Method and system for simultaneous measurement of strain and temperature utilizing dual core fiber

ABSTRACT

There is provided a system for measuring temperature and strain simultaneously utilizing Brillouin Scattering within an optical fiber. The system has a cladding, a first optical core within the cladding and a second optical core within the cladding and having a different refractive index profile and/or composition than the first core. Means to couple light into and out of said individual optical cores and/or from one optical core to the other within the fiber is provided along with means for calculating strain and temperature characteristics based on measured Brillouin frequencies for said optical cores.

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation Application of pending U.S. application Ser. No.15/236,000 filed Aug. 12, 2016, which is a Continuation Application ofissued U.S. application Ser. No. 141705,424 filed May 6, 2015, whichclaims the benefit of Canadian Application Serial No. 2,851,047 filedMay 6, 2014.

TECHNICAL FIELD

The present invention relates to a method and system for thesimultaneous measurement of strain and temperature utilizing a dual corefiber.

BACKGROUND OF THE INVENTION

Co-owned U.S. Pat. No. 7,599,047 B2 describes a method and a system thatutilizes a pair of fibers connected or installed together, with one ofthe fibers having a refractive index profile or waveguide compositionthat differs from that of the other fiber. By measuring the Brillouinfrequency shift along each of the fibers and by determining coefficientsof Brillouin frequency shift versus strain and temperature for each ofthe fibers, one can measure strain and temperature along the fiberlengths.

The main drawback of this patent for field applications is that thefibers need to be installed with great care, to ensure both fibersexperience the same strains and temperature at matching points. If onefiber is not experiencing the same conditions as the other fiber alongthe measurement region of interest, the measured strain and temperaturevalues will be incorrect.

SUMMARY

In accordance with one aspect of the present invention, there isprovided an optical fiber comprising a cladding and a first core withinthe cladding. A second core within the cladding has a differentrefractive index than the first core.

In accordance with another aspect of the present invention, there isprovided a system for measuring temperature and strain simultaneouslyutilizing Brillouin Scattering within an optical fiber comprising acladding and a first optical core within the cladding. A second opticalcore within the cladding has a different refractive index profile and/orcomposition than the first core.

Means to couple light into and out of the individual optical coresand/or from one optical core to the other within the fiber is providedalong with means for calculating strain and temperature characteristicsbased on measured Brillouin frequencies for the optical cores.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will be better understood with reference tothe drawings in which:

FIG. 1 shows a dual core fiber structure;

FIG. 2A shows the use of a dual core fiber with one laser and areflector;

FIG. 2B shows the use of a dual core fiber with two lasers and areflector;

FIG. 3 shows use of a dual core fiber using lasers at opposite ends;

FIG. 4 shows a fiber reflector arrangement; and

FIG. 5 shows a dual core fiber launch arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention consists of a special fiber that has two cores 2, 4 withinthe same cladding 6 forming separate waveguides, as shown in FIG. 1. Thefiber is constructed so that the two cores 2, 4 have differentrefractive index profiles and/or compositions. Therefore each core willhave different coefficients for Brillouin frequency shift versus strainand temperature. Because both waveguides are within the same fibercladding 6, one can easily install the fiber in the field. Bothwaveguides experience the same temperature and strains because they arewithin the same cladding 6. The cladding shape also matches that ofconventional fiber, which also aids in its installation.

FIGS. 2A and 2B show two practical examples of how to use the fiber inthe field. In FIG. 2A laser light 8 is launched via a circulator orsplitter 10 into one fiber core on one end. This connection can be doneby aligning the core from the splitter to one core of the fiber. At theother end the light is reflected via reflector 12 from core 1 to core 2.Brillouin scattered light is reflected within both cores along the fiberlength and is gathered by a receiver 14 for analysis. In FIG. 2B asecond laser 16 launches light into the second fiber core, with thelasers functioning as pump and probe lasers respectively.

FIG. 3 shows another example of how the invention is used.Circulators/splitters 18 connect lasers 20, 22 and receivers 24, 26 todifferent fiber cores from opposite ends. The lasers interrogate eachfiber core separately and the Brillouin spectra is received from eachcore for analysis. Similarly, an additional pair of lasers can be addedusing the method shown in FIG. 2B, to act as pump and probe lasers foreach fiber core.

FIG. 4 illustrates one possible method to reflect light from one fibercore to the other. The end of the fiber is placed at the focal plane ofa lens 28. The lens 28 collimates light from the first core. Thecollimated beam is then reflected by a mirror 30 back through the lens28, which focuses the light into the second core.

FIG. 5 illustrates one possible method to launch light into both fibercores simultaneously. The light from each input fiber 38, 40 iscollimated by lenses 32, 34, and then directed through a combining lens36 to focus the light onto each core. The arrangement can also be usedin the reverse direction to direct the light from each core intoseparate fibers.

Other techniques may be used to provide the functions described in FIGS.4 and 5.

The scope of the claims should not be limited by the preferredembodiments set forth in the examples given above, but should be giventhe broadest interpretation consistent with the description as a whole.

What is claimed:
 1. A system for measuring temperature and strainsimultaneously utilizing Brillouin scattering within an optical fibercomprising: a cladding; a first optical core within the cladding; asecond optical core within the cladding having a different refractiveindex profile and composition than the first optical core; and areflector having a lens configured to collimate light from the firstoptical core and a mirror configured to reflect the collimated lightback through the lens into the second optical core to couple light intoand out of said individual optical cores and from one optical core tothe other within the fiber.
 2. The system of claim 1 further comprisinga laser configured to send a light through a splitter or circulator anda receiver for receiving Brillouin scattered light through the splitteror circulator.
 3. The system of claim 2 further comprising a receiverconfigured to receive Brillouin scattered light, wherein the receiver isconfigured to measure the Brillouin frequency for the first and secondoptical cores
 4. The system of claim 3 wherein the receiver isconfigured to calculate strain and temperature characteristics based onthe measured Brillouin frequencies for the first and second opticalcores.
 5. A system for measuring temperature and strain simultaneouslyutilizing Brillouin scattering within an optical fiber comprising: acladding; a first optical core within the cladding; a second opticalcore within the cladding having a different refractive index profile andcomposition than the first optical core; a reflector having a lensconfigured to collimate light from the first optical core and a mirrorconfigured to reflect the collimated light back through the lens intothe second optical core to couple light into and out of said individualoptical cores and from one optical core to the other within the fiber;and a first lens configured to collimate light from a first source, asecond lens configured to collimate light from a second source and acombining lens configured to focus the collimated light from the firstand second sources to the first and second optical cores.
 6. The systemof claim 5 further comprising a laser configured to send a light througha splitter or circulator and a receiver for receiving Brillouinscattered light through the splitter or circulator.
 7. The system ofclaim 6 wherein the splitter is aligned to the first lens to send lightinto the first optical core and further comprising a second laseraligned to the second lens for sending light into the second opticalcore.
 8. The system of claim 7 wherein the lasers function as pump andprobe lasers.
 9. The system of claim 8 further comprising a receiverconfigured to receive Brillouin scattered light, wherein the receiver isconfigured to measure the Brillouin frequency for the first and secondoptical cores
 10. The system of claim 9 wherein the receiver isconfigured to calculate strain and temperature characteristics based onthe measured Brillouin frequencies for the first and second opticalcores.
 11. A system for measuring temperature and strain simultaneouslyutilizing Brillouin scattering within an optical fiber comprising: acladding; a first optical core within the cladding; a second opticalcore within the cladding having a different refractive index profile andcomposition than the first optical core; and a first laser and receiverconnected through a first splitter or circulator to one end of the firstoptical core and a second laser and receiver connected through a secondsplitter or circulator to an end of the second optical core that isopposite the one end of the first optical core.
 12. The system of claim11 wherein the first receiver is configured to measure the Brillouinfrequency for the first optical core and the second receiver isconfigured to measure the Brillouin frequency for the second opticalcore.
 13. The system of claim 12 wherein the first and second receiversare configured to calculate strain and temperature characteristics basedon the measured Brillouin frequencies for the first and second opticalcores.
 14. A system for measuring temperature and strain simultaneouslyutilizing Brillouin scattering within an optical fiber comprising: acladding; a first optical core within the cladding; a second opticalcore within the cladding having a different refractive index profile andcomposition than the first optical core; a lens arrangement on one endof the fiber consisting of a first lens configured to collimate lightfrom a first source, a second lens configured to collimate light from asecond source and a combining lens configured to focus the collimatedlight a first and second sources to the first and second optical coresrespectively; and a lens arrangement on the opposite end of the fiberconsisting of a first lens configured to collimate light from a thirdsource, a second lens configured to collimate light from a fourth sourceand a combining lens configured to focus the collimated light from thethird and fourth sources to the second and first optical coresrespectively.
 15. The system of claim 14 further comprising a laserconfigured to send a light through a splitter or circulator and areceiver for receiving Brillouin scattered light through the splitter orcirculator.
 16. The system of claim 15 wherein the splitter is alignedto the first lens to send light into the first optical core and furthercomprising a second laser aligned to the second lens for sending lightinto the second optical core.
 17. The system of claim 16 furthercomprising of a third laser, splitter or circulator, receiver and fourthlaser, attached to the opposite end of the fiber in the same manner asdescribed in claims 15 and 16, with the difference being that the thirdlaser and receiver launches light into the second optical core, whilethe fourth laser is launched into the first optical core.
 18. The systemof claim 17 wherein the first and fourth lasers function as pump andprobe lasers for the first optical core, while the third and secondlasers function as pump and probe lasers for the second optical core,19. The system of claim 18 wherein the first receiver is configured tomeasure the Brillouin frequency for the first optical core and thesecond receiver is configured to measure the Brillouin frequency for thesecond optical core.
 20. The system of claim 19 wherein the first andsecond receivers are configured to calculate strain and temperaturecharacteristics based on the measured Brillouin frequencies for thefirst and second optical cores.